Consolidate templated source tests
Description
Consolidate tests which run against a range of different source types into a single location. This is preparation work for #944 (closed).
This is probably easiest to review commit by commit.
Tests have been moved from the following files:
- frontend/mirror.py
- frontend/fetch.py
- frontend/buildcheckout.py
- frontend/track_cross_junction.py
- frontend/track.py
- integration/source-determinism.py
Edited by Phil Dawson